Measurement of thermal structures in a turbulent Rayleigh-Benard system of large aspect ratio using luminophores

Abstract

Rayleigh-B\'enard convection (RBC) where a horizontal fluid layer of height $H$ is heated from below and cooled from above is an archetypical model system for studying pattern formation under small thermal driving as well as to study turbulence under strong thermal driving. If the aspect ratio $\Gamma = L/H$ between the lateral dimension $L$ and the height is large enough, also under turbulent conditions do large scale flow pattern occur that to some extent resemble the laminar convection rolls at small Rayleigh numbers $\Ra$. Studying the dynamics of these coherent superstructures in direct numerical simulation (DNS) usually gives a very detailed view of the flow field, but only over short time intervals, whereas experimental studies usually provide only localised information but over longer times. Here, we present results from temperature measurements close to the top plate using temperature sensitive paint (TSP). This is a chemical where a luminescent complex compound is embedded in a poly-urethane matrix. A thin layer of TSP was applied on the top plate of a rectangular large aspect ratio RBC cell (fig.~\ref{fig:SQR_setup}a and b). The cell was filled with water (Prandtl number \Pran=7.0), is heated via a carbon fibre fabric and cooled on top via temperature regulated water. The TSP is illuminated by four UV-LED ($\lambda_{ab} \approx 395$\,nm) from the top and emits light at $\lambda_{em}\approx 620$\,nm. Since the emission intensity decreases monotonically with temperature, by using a proper calibration function, we can calculate the temperature at the top boundary layer (see e.g., fig.~\ref{fig:SQR_setup}c). The thin blue lines are the cold plumes that detach from the top boundaries, while the red circular patches are the warm plumes rising from the bottom. Measuring the instantaneous temperature field for a long time allows to investigate the statistics of temperature distribution as well as the relation between the small scale plumes and the large scale turbulent superstructures. In this talk we present statistical analysis of the temperature fluctuations for convection cells at $\Gamma = 16$ and $\Gamma =32$ as well as \Ra\ in the range $1.9\times 10^5\le \Ra\le 5.7\times 10^6$. This work was funded by the priority program Turbulent Superstructures (SPP 1881) of the Deutsche Forschungsgemeinschaft (DFG) - 429432497.